The invention relates to a frequency converter designed for receiving an input signal having an intermediate frequency and a signal originating from an oscillator and having an oscillation frequency, and for supplying an output signal having a radio frequency whose value is equal to the absolute value of the difference between the oscillation frequency and the intermediate frequency.
Such a frequency converter is known from the specification of the RF2640 circuit manufactured by the RF Micro-Devices Company.
This circuit is designed for generating an output signal of asymmetrical character on the basis of an input signal of a differential character, the signal originating from the oscillator also being of an asymmetrical character. The output signal is the result of a multiplication operation between the input signal and the signal coming from the oscillator. If the output signal of such a frequency converter comprises a component whose frequency is equal to the difference between the oscillation frequency and the intermediate frequency, it will also comprise a parasitic component whose frequency is equal to the sum of the oscillation frequency and the intermediate frequency, this sum being referred to here as the image frequency. The output signal of the frequency converter carries a frequency information which is then split between the radio frequency and the image frequency, whereas a receiver of this information will be configured for receiving the radio frequency only. The parasitic component should accordingly be suppressed so that only the component of the output signal of the frequency converter having the radio frequency will contain the useful information to be transmitted, because otherwise the transmitted signal will be useless for the receiver for which this information is destined. This requires the use of a filtering device in addition to the frequency converter. Besides, the gain and the linearity of the known frequency converter are insufficient for a number of applications and have to be improved by means of components external to the converter, such as microstrip lines, which further complicate the use of the known converter.
The invention has for its object to counteract the above disadvantages by providing a frequency converter which is capable of realizing internally a satisfactory rejection of the image frequency while offering a good performance as regards noise and gain and a linearity which is improved with respect to the known converter.
According to the invention, a frequency converter as described in the opening paragraph for this purpose comprises:
a first quadratic module designed for receiving the output signal from the oscillator and for supplying a first and a second output signal in mutual phase quadrature, both having the oscillation frequency as their frequency,
a second quadratic module designed for receiving the input signal of the converter and for supplying a first and a second output signal in mutual phase quadrature, both having the intermediate frequency as their frequency,
a first mixer designed for supplying an output signal which represents a product of the first output signals of the first and second quadratic modules,
a second mixer designed for supplying an output signal representing a product between the second output signals of the first and second quadratic modules, and
combination means for recombining the output signals of the first and second mixers.
The advantages of the invention may be readily understood from the ensuing example, in which the input signal of a frequency converter according to the invention is given by the expression IS=sin(xcfx89FIxc2x7t), while the output signal of the oscillator is given by the expression OS=sin(xcfx89LOxc2x7t). In this example, the first and the second quadratic module are arranged so as to generate respective first and second signals (FI1; FI2) and (LO1; LO2), which are written as follows:
FI1=sin(xcfx89FIxc2x7t+Π) 
FI2=sin(xcfx89FIxc2x7t+3Π/2) 
LO1=sin(xcfx89LOxc2x7t+Π/4) 
LO2=sin(xcfx89LOxc2x7t+3Π/4) 
The output signals RS1 and RS2 of the first and second mixers resulting from the multiplication operations carried out by said mixers are then written as follows, using conventional trigonometric formulas:
RS1=(sin((xcfx89LOxe2x88x92xcfx89FI)xc2x7t+Π/4))/2xe2x88x92(sin((xcfx89LO+xcfx89FI)xc2x7t+Π/4))/2, and 
RS2=(sin((xcfx89LO+xcfx89FI)xc2x7t+Π/4))/2. 
After recombination of said output signals, the frequency converter delivers a signal which represents the sum of the output signals of the first and second mixers, which is thus written as RS=sin((xcfx89LOxe2x88x92xcfx89FI)xc2x7t+Π/4)=sin(xcfx89RFxc2x7t +Π/4), and which accordingly only comprises a component having the radio frequency equal to xcfx89RF/2Π. The frequency converter according to the invention thus automatically realizes a good rejection of the image frequency, because no sin((xcfx89LO+xcfx89FI)xc2x7t) component is present in the output signal of the frequency converter.
In an embodiment of the invention, each mixer comprises two differential pairs forming a Gilbert cell whose conduction is intended to be controlled by one of the output signals of the first quadratic module, and which pairs are designed to be baised by means of one of the output signals of the second quadratic module.
This embodiment is simple and uses tried and tested elements. Moreover, the biasing of the Gilbert cells by means of the output signals of the second quadratic module enables to reduce to a minimum the interface between the input signal of the frequency converter and the mixers, and thus to limit the noise generated by the multiplication of the input signal by the signal originating from the oscillator.
In a particular embodiment of the invention, the second quadratic module is provided with a transconductance stage enabling it to transform a voltage information carried by the input signal of the converter into a current information.
This embodiment allows a maximum simplification of the interface between the second quadratic module and the mixers if the input signal is formed by a voltage, which is usually the case in the present state of the art. The use of a transconductance stage furthermore permits to obtain a good linearity for the multiplications carried out by the mixers.
In a modification of this embodiment, the second quadratic module includes a polyphase filter enabling the production of a plurality of currents out of phase with one another and representative of the information carried by the input signal.
In a particularly advantageous embodiment of the invention, the second quadratic module and the first and the second mixer are incorporated within a same integrated circuit.
The integration indicated above allows a reduction in the connection line lengths between the various integrated elements, and thus a reduction of the noise generated by the frequency converter to a minimum.
The frequency converter according to the invention may be used in any type of application where a frequency conversion is required, particularly frequency shifts before transmission of radio wave signals. In one of these embodiments, the invention also relates to a wireless telephony device comprising a signal processing unit signal and an antenna designed for transmitting a radio frequency signal, characterized in that it comprises in addition a frequency converter as described above and designed for receiving its input signal from the signal processing unit, while the output signal of said frequency converter forms the radio frequency signal.